1. Field of the Invention
The present invention relates to an image reading apparatus for reading an image recorded on an original such as a film.
2. Description of the Related Art
In recent years, the following technique has developed. That is, an image obtained by reading a film image serving as an original by a film scanner or the like is displayed on a display, and a user edits and processes the image to obtain a preferable image. The frequency of transmission of image data of a film image to a communication destination by using a communication device rapidly increases. As a method of reading a film image, the following methods are generally used. That is, a film original is irradiated by an illumination light source from one side of the film original to directly read a transparent film image from an area sensor, or a film image is read by causing a high-density line sensor to mechanically scan the film image.
Here, as an example of a conventional image reading apparatus, a film scanner for reading the image of a transparent original (film) will be exemplified. A conventional film scanner which reads an image recorded on a film to input the image to a personal computer has been mainly developed for a 35 mm standard film. When an image is to be read by such a film scanner, scanning (pre-scanning) at a low resolution, and an image obtained by this scanning is displayed on a personal computer as a preview screen. Thereafter, the film scanner is instructed to main-scan a desired read range designated on the preview screen. The film scanner main-scans the desired range of the indicated image to transmit image data obtained by this scanning to the personal computer.
FIG. 30 is a schematic view showing the arrangement of a conventional film scanner. Referring to FIG. 30, reference numeral 1101 denotes an original table (film carriage), and reference numeral 1102 denotes a developed film which is fixed to the original table 1101. Reference numeral 1103 denotes a lamp serving as a light source; 1104, a mirror; 1105 and 1106, lenses; 1107, a line sensor constituted by a CCD (Charge Coupled Device); and 1108, an aperture unit whose aperture can be mechanically adjusted.
Reference numeral 1109 denotes a motor for moving the original table 1101 in a scanning direction; 1110, a motor for adjusting the aperture of the aperture unit 1108; 1111, a control circuit; and 1113, a sensor for detecting the position of the original table 1101.
The lamp 1103, the line sensor 1107, the motors 1109 and 1110, and the sensor 1113 are electrically connected to the control circuit 1111, and the control circuit 1111 controls determination and adjustment of an aperture, movement of the original table 1101, ON/OFF of the lamp 1103, and the like. Light irradiated from the lamp 1103 passes through the developed film 1102 along an optical axis 1112, is reflected by the mirror 1104, and then passes through the lenses 1105 and 1106 and the aperture unit 1108 to reach the line sensor 1107.
A method of reading image information from the developed film 1102 by using the film scanner 30 will be described below.
The control circuit 1111 causes the motor 1110 to drive the aperture unit 1108 such that the aperture of the aperture unit 1108 is set in a predetermined standard state, and turns on the lamp 1103. The control circuit 1111 causes the sensor 1113 to detect the position of the original table 1101, rotates the motor 1109 to scan the image of the developed film 1102, and then moves the original table 1101 toward the film surface. The control circuit 1111 causes the line sensor 1107 to read light transmitted through the developed film 1102.
Here, the control circuit 1111 determines the aperture of the aperture unit 1108 not to saturate electric charges in the light-receiving element of the line sensor 1107, and causes the motor 1110 to adjust the aperture of the aperture unit 1108 to the determined aperture. Upon completion of the adjustment, the original table 1101 is moved by the motor 1109 toward the film surface to scan the video image range of the film 1102, and light emitted from the lamp 1103 and passing through the film 1102 is received by the line sensor 1107. A video signal output from the line sensor 1107 in accordance with the received light is transmitted to the control circuit 1111. Upon completion of the scanning for the video image range of the film 1102, the control circuit 1111 turns off the lamp 1103 and drives the motor 1109 to return the original table 1101 to an initial position.
With the above procedure, the film scanner reads the image information of the film 1102 fixed to the original table 1101. The read image information can be used in an external device such as a personal computer.
However, on the optical path used when the light emitted from the lamp 1103 reaches the line sensor 1107, the aperture unit 1108 is arranged, and the aperture of the aperture unit 1108 is adjusted to adjust an amount of light being incident on the line sensor 1107. For this reason, the following drawbacks are generated.
Since the aperture unit 1108 for adjusting an amount of light being incident on the line sensor 1107 must be arranged, the film scanner has a complex structure. In addition, since the film scanner must comprise the control circuit for controlling the operation of the motor 1110, the film scanner cannot be easily reduced in size.
An image signal output from the CCD includes random noise and noise generated by a variation in dark current. FIGS. 31A and 31B show image signal outputs from the CCD in a state wherein incident light is interrupted. FIG. 31A shows a dark current output from the CCD at 25.degree. C. for an accumulation time of 10 msec, and the variation in dark current is about 4 mV.
This dark current has a value changed depending on a temperature and an accumulation time. For example, FIG. 31B shows an image signal output from the CCD when the temperature and the accumulation time in the state in FIG. 31A increase by .alpha..degree. C. and .beta. msec, respectively. It is understood that the level of the dark current is high as a whole.
Due to such a dark current, the ratio of an image signal and noise of an image of a high-density portion of, especially, a negative film or the like has an equal level. For this reason, when the image is displayed, stripes may be formed in a sub-scanning direction. Therefore, in the prior art, an image process such as smoothing or averaging is performed to an image signal to make the stripes inconspicuous. However, since the sharpness of the image is degraded, image quality is degraded.
As an illumination light source of the film original reading apparatus, a halogen lamp, a fluorescent lamp, an LED, and the like are used. These lamps are selectively used depending on applications. For example, a high-speed reading operation is to be performed, a halogen lamp, a fluorescent lamp, or the like is used as a brighter light source to increase illuminance on a film surface. In addition, when the apparatus is desired to be reduced in size, an LED may be used as a light source which generate a small amount of heat.
However, when a halogen lamp or a fluorescent lamp is used as a light source, the light source generate a large amount of heat. For this reason, in-focus precision is degraded by waviness generated on a film surface by the heat, or the film itself may be damaged. In order to solve this problem, the interval between the light source and the film is increased such that the film is not adversely affected by heat from the light source, or a cooling fan is arranged. As a result, the apparatus cannot be easily reduced in size. In addition, since a time from the light source is turned on to when an amount of light is stable is long, the ON state of the light source must be kept before the film image is read regardless of a reading operation.
On the other hand, when an LED is used as a light source, illuminance on the film surface decreases because the absolute luminance of the light source is low, and a line sensor or an area sensor takes a long accumulation time. For this reason, a reading speed can not be increased disadvantageously.